Canine herpesvirus (CHV) is an important cause of the deaths of neonatal pups; however, the disease caused by this virus is inapparent in older animals; Carmichael, J. Am. Vet. Med. Assoc. 156: 1714-1721 (1970); Carmichael et al, Am. J. Vet. Res., 26: 802-814 (1965). The dog is the only known host, and viral growth occurs exclusively in cells of canine origin; Carmichael et al, Proc. Soc. Exp. Biol. Med., 120: 644-650 (1966). Growth of CHV is markedly restricted by the temperature of incubation, the optimal range being 35.degree. to 37.degree. C.; Aurelian, Am. J. Vet. Res., 30: 1945-1952 (1968); Carmichael et al, J. Am. Vet. Med. Assoc., 120: 664-668 (1970). Plaque characteristics of several field isolates grown at optimal temperatures in primary dog kidney cell (DKC) cultures have appeared consistent in size and character (Pryde et al, Vet. Rec., 79: 660-661 (1966); Spertzel et al, Proc. Soc. Exp. Biol. Med., 120: 651-655 ( 1965)), with the unique exception of the BR strain isolated in England from the genital tract of dogs in vesicular lesions; Poste, Arch. Gesamte Virusforsch., 36: 147-157 (1972). The BR strain of CHV was unusual in that it caused plaques on DKC monolayers consisting principally of polykaryocytes. Unfortunately, it was apparently not preserved in a viable state.
The recognition of natural plaque mutants of several animal viruses has facilitated the exploration of viral genetics; in some instances plaque characteristics also have been associated with differences in strain virulence and have been exploited for vaccine; Darlington et al, The Herpesviruses (Kaplan, ed.) Academic Press Inc., New York, 1973, pages 93-132; Takemoto, Prog. Med. Virol., 8: 314-348 (1966). Herpesvirus plaque morphology has been used as a biological marker for genetic studies (Arlington et al and Takemoto, supra), as well as for discriminating closely related strains; Cho, Avian Dis., 20: 324-331 (1976); Lancz, Arch. Gesamte Virusforsch.; 46: 31-43 (1974); Monk et al, Arch. Gesamte Virusforsch.; 37: 308-315 (1972). Plaque characteristics of Marek's disease herpesvirus (MDHV) in natural host cells also have proved useful in the preliminary assessment of strain virulence; Biggs et al Oncogenesis and Herpeviruses (Biggs et al ed.), WHO International Agency for Research on Cancer, Lyon (1972) Pages 88-94; Cho, Avian Dis., 20: 324-331 (1976).
Herpesvirus virulence has been associated with a variety of other in vitro properties, including the type of cytopathology, viral growth characteristics at difference temperatures, antigenic differences, host cell range, resistance to 5-iododeoxyuridine, and plaque characteristics in alien host cells; Cho, supra; Darlington, supra; Koment et al, Intervirology, 5: 10-20 (1975); Pryde, supra; Tokumaru, Proc. Soc. Exp. Biol. Med., 96: 55-58 (1957). Herpesvirus homines (HVH) types have been studied extensively. With this virus, differences in plaque morphology have permitted the clear discrimination between strains; Monk et al, supra; Roizman, Virology; 15: 75-79 (1961). An HVH-1 strain that produced small plaques in rabbit kidney cells also was found to be less virulent for rabbits and mice than the wild-type (large plaque) virus; Rapp et al, Proc. Soc. Exp. Biol. Med., 166: 361-365 (1964).